2 Items which are forgotten very easily in routine maintenance; the coolant expansion reservoir cap and the oil filler cap. The coolant cap is spring loaded cap (mechanical component (!)), which is usually set to 1.1 BAR, to allow the coolant to vent whenever the pressure is too high. But when it vents too fast (spring lost tension), the coolant system is not able to operate efficiently.
Therefore its a good habit of changing these out every now and then. For the couple of euros, it should not be forgotten.
I also swapped out the oil filler cap, as this was leaking/seeping out a little bit of oil.
All CLS55 AMG pictures here in my Google Photos album: https://photos.app.goo.gl/6SP5PFAUXCqanoSX9